By metric name, the value that plot_daily_pattern() colors
white, with lower values in blue and higher in red: room temperature, the
middle of the comfortable range of humidity (40 to 60\
atmospheric pressure at sea level, and for particles the daily guideline of
the World Health Organization (who_guidelines). NA for the metrics with
no such value (light, proximity, the gas sensors, whose resistance is only
relative, and PM1, which has no guideline): these keep blue_steps.